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6 (there are no pull-up resistors), and may cause ICC increases. Connect unused inputs to GND for the best performance. Receivers The ISL83386E contains standard inverting receivers that convert RS-232 signals to CMOS output levels and accept inputs up to ±25V while presenting the required 3kΩ to 7kΩ input impedance (see Figure 1) even if the power is off (VCC = 0V). The receivers’ Schmitt trigger input stage uses hysteresis to increase noise immunity and decrease errors due to slow input signal transitions. Receiver outputs swing from GND to VL, and tristate in powerdown. Low Power Operation This 3V device requires a nominal supply current of 0.3mA, even at VCC = 5.5V, during normal operation (not in powerdown mode). This is considerably less than the 11mA current required by comparable 5V RS-232 devices, allowing users to reduce system power simply by replacing the old style device with the ISL83386E in new designs. Powerdown Functionality The already low current requirement drops significantly when the device enters powerdown mode. In powerdown, supply current drops to 1 µA, because the on-chip charge pump turns off (V+ collapses to VCC, V- collapses to GND), and the transmitter and receiver outputs tristate. This micro- power mode makes these devices ideal for battery powered and portable applications. Software Controlled (Manual) Powerdown The ISL83386E may be forced into its low power, standby state via a simple shutdown (SHDN) pin (see Figure 2). Driving this pin high enables normal operation, while driving it low forces the IC into its powerdown state. The time required to exit powerdown, and resume transmission is less than 100 µs. Connect SHDN to VCC if the powerdown function isn’t needed. VL Logic Supply Input Unlike other RS-232 interface devices where the CMOS outputs swing between 0 and VCC, the ISL83386E features a separate logic supply input (VL; 1.8V to 5V, regardless of VCC) that sets VOH for the receiver outputs. Connecting VL to a host logic supply lower than VCC, prevents the ISL83386E outputs from forward biasing the input diodes of a logic device powered by that lower supply. Connecting VL to a logic supply greater than VCC ensures that the receiver output levels are compatible even with the CMOS input VIH of AC, HC, and CD4000 devices. Note that the VL supply current increases to 100 µA with V L = 5V and VCC =3.3V inputs, thereby setting their switching thresholds to levels compatible with the logic supply. This separate logic supply pin allows a great deal of flexibility in interfacing to systems with different logic supplies. If logic translation isn’t required, connect VL to the ISL83386E VCC. Capacitor Selection The ISL83386E charge pumps only require 0.1 µF capacitors for the full operational voltage range. Table 3 lists other acceptable capacitor values for various supply voltage ranges. Do not use values smaller than those listed in Table 3. Increasing the capacitor values (by a factor of 2) reduces ripple on the transmitter outputs and slightly reduces power consumption. TABLE 2. POWERDOWN TRUTH TABLE SHDN INPUT TRANSMITTER OUTPUTS RECEIVER OUTPUTS MODE OF OPERATION L High-Z High-Z Manual Powerdown H Active Active Normal Operation RXOUT GND ≤ VROUT ≤ VL 5k Ω RXIN -25V ≤ VRIN ≤ +25V GND VL FIGURE 1. RECEIVER CONNECTIONS TABLE 3. REQUIRED CAPACITOR VALUES VCC (V) C1 ( µF) C2, C3, C4 ( µF) 3.0 to 3.6 0.1 0.1 4.5 to 5.5 0.047 0.33 3.0 to 5.5 0.22 1 FIGURE 2. CONNECTIONS FOR MANUAL POWERDOWN PWR SHDN CPU I/O ISL83386E MGT LOGIC UART VL I/O CHIP POWER SUPPLY VCC ISL83386E |
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